Short spacing under 47 CFR 90.621, and when you need a concurrence letter

Distance between fixed stations

Stations on the channels in Part 90, subpart S (the 800 and 900 MHz bands) are assigned co-channel frequencies solely on the basis of distance between fixed stations (47 CFR 90.621(b)). Two groups are outside that: EA-based and MTA-based SMR systems, and the 806-809/851-854 MHz public safety channels, which are assigned and protected under the regional plans (47 CFR 90.621(g)).

Protection has to be earned. A conventional system in the Public Safety or Business/Industrial/Land Transportation categories that has not met the loading needed for channel exclusivity is not afforded co-channel protection at all (47 CFR 90.621(c)).

The 70-mile baseline

An existing station is protected out to 113 km (70 mi). Inside that distance a new co-channel station has three routes: qualify under the short-spacing table, bring letters of concurrence, or seek a waiver backed by an interference analysis (47 CFR 90.621(b)(4) and (b)(5)).

The short-spacing table

On request, co-channel stations may be separated by less than 113 km, down to a minimum of 88 km (55 mi), by meeting the power and height combinations in the table in 47 CFR 90.621(b)(4). The table is read with three numbers: the proposed station's ERP, its DHAAT toward the existing station, and the existing station's DHAAT toward the proposed one. Where a value is not in the table, the next higher one is used, and every existing station is assumed to run 1000 W ERP.

The distances come from the R-6602 propagation curves, and they encode one test: the proposed station's 22 dBu F(50,10) interference contour must not overlap the existing station's 40 dBu F(50,50) service contour. The reverse pair is not considered.

Two readings of the table show its range. A proposed station at 1000 W and 305 m DHAAT gets no reduction at all against an existing station at 305 m: 113 km. A proposed station at 250 W and 108 m DHAAT can sit 96 km from an existing station whose DHAAT toward it is 150 m, and 88 km from one at 75 m or lower.

The application states that it is filed under the table and lists every co-channel station within 113 km with its DHAAT and ERP, alongside the proposed station's. The table distances are 56 km (35 mi) greater for stations on Santiago Peak, Sierra Peak, Mount Lukens and Mount Wilson in California and on the Washington sites the rule lists.

DHAAT, not HAAT

The table uses directional height above average terrain: the average of the antenna's height above average terrain from 3 to 16 km (2 to 10 mi) out, along the radial toward the other station and the radials 15 degrees either side of it (47 CFR 90.621(b)(4)(i)). It is a height in one direction, so the same site can have a very different DHAAT toward two different incumbents.

Where either station's DHAAT is 458 m (1500 ft) or more, the separation grows: take the distance for 305 m and add 1.6 km (1 mi) for every 30.5 m (100 ft), or part of it, above 458 m, figured separately for each station and added together (47 CFR 90.621(b)(4)(ii)).

Closer than the table: concurrence or waiver

A station can be closer than the table allows if the applicant submits letters of concurrence in which the applicant and each co-channel licensee within the separation agree to accept any interference the reduced spacing causes (47 CFR 90.621(b)(5)). Each letter must certify that the concurring licensee's system is constructed and fully operational, and the applicant serves each concurring licensee with a copy of the application.

Without concurrence, a station closer than the table needs a waiver. The application carries an interference analysis, on any generally accepted terrain-based propagation model, showing the co-channel stations get the same or greater protection than the table gives; below 88 km it adds an analysis of mobile transmitters into existing base receivers. Every co-channel licensee in the area is served with the application and may oppose it within 30 days (47 CFR 90.621(b)(4)).

Adjacent channels in 809-817/854-862 MHz

On the adjacent channels of the 809-817/854-862 MHz segment, separation is a contour test instead, unless the stations already meet the co-channel distance criteria (47 CFR 90.621(d)). The forward analysis checks that the applicant's interference contour does not overlap an adjacent-channel incumbent's 40 dBu F(50,50) contour, with the incumbent assumed at its maximum permitted ERP. The reciprocal analysis checks the incumbent's interference contour against the applicant's 40 dBu contour. The interference contour levels come from Tables 4 and 5, by channel bandwidth and modulation.

Overlap under either analysis can be cured by a letter of concurrence from each incumbent involved: in the forward case the incumbent accepts the interference, and in the reciprocal case it states it does not object to the applicant receiving it (47 CFR 90.621(d)(4)).

At VHF and UHF, contours instead

Centralized trunking in the 150-174 MHz and 421-512 MHz bands is screened by contours rather than a distance table. The affected incumbents are those with both a spectral overlap and a contour overlap: the proposed station's interference contour (19 dBu at VHF, 21 dBu at UHF) reaching the incumbent's service contour (37 dBu at VHF, 39 dBu at UHF) (47 CFR 90.187(d)). The application carries either written consent from every affected licensee or a coordinator's certification that there are none.

A concurrence letter is an agreement between licensees to accept interference. It does not make the interference go away, and it is only as good as the facilities it describes, which is why 47 CFR 90.621(b)(5) asks the concurring licensee to certify its system is built and operating.